This invention relates generally to sensors for measurement of blood flow parameters and more specifically to the use of such sensors in conjunction with implantable medical devices.
In a number of medical and surgical procedures, it is desirable to monitor blood parameters, in the course of diagnostic testing or monitoring of cardiovascular system performance. Blood parameters may also be employed to control the operation of an implantable device. For example, it is known that a relationship exists between central venous blood pressure and heart rate and that blood pressure may be employed to regulate pacing rate. Similarly, central venous oxygen saturation, Ph, temperature and other blood parameters have all been employed to regulate pacing rate.
It is also known that high rate ventricular tachycardias and ventricular fibrillation have marked effects on hemodynamic performance and that the compromise in hemodynamic performance may be measured in either the arterial or venous system. Blood parameters are recognized as being useful in this context to assist in diagnosing and monitoring of tachyarrhythmias and in controlling the operation of anti-tachycardia devices.
The direct measurement of arterial blood flow parameters using sensors introduced into the left heart or the arterial system has been proposed, but is not presently practiced in the context of chronically implantable monitoring or therapeutic devices. The primary concern with chronic implant of a sensor in the arterial system is that the introduced sensor may provoke thrombus formation and/or embolization due to the difficulty of providing a high pressure seal at the point of entry to the heart chamber or artery. Moreover, chronic implantation often results in the accumulation of fibrotic material on the implanted device, raising the possibility of obstruction of the artery. In order to avoid these problems, the use of a perivascular transducer placed adjacent to or around the exterior vascular wall of the selected artery has been proposed. However, such a device may itself poses the risk of erosion and rupture of the artery or constriction of the artery either directly or by tissue growth around the transducer. Thus, most proposals or using chronically implanted sensors have focused on sensors located in the venous system.
U.S. Pat. Nos. 4,899,751 and 4,967,749 issued to Cohen disclose pressure sensors located in a variety of locations within the venous and arterial systems. The Cohen patents disclose the use of measured blood pressure to control implantable devices such as cardiac pacemakers or anti-tachycardia devices. Similarly, U.S. Pat. No. 4,967,748, issued to Cohen discloses the use of oxygen saturation sensors, located in the vascular system for controlling operation of implantable antitachycardia devices. U.S. Pat. No. 4,750,495 discloses the use of oxygen saturation sensors for controlling operation of implantable pacemakers.
The article "Perivascular Impedance Sensors for In Vivo Chronic Blood Measurement: Detection Systems for Automatic Defibrillators, Cardioverters and Blood Pressure Controllers" by Tacker et al., published in the proceedings of the 37th ACEMB, September, 1984, page 20 and the article "A New Implantable Arterial Blood Sensor for Detection of Ventricular Fibrillation" by Konrad et al, published in Medical Instrumentation, December, 1988, Vol. 22(6): 304-311 both disclose sensors for measurement of arterial blood flow to the brain. The Tacker and Konrad articles suggest that placing the transducer within the arterial system as suggested in the Cohen patent is disadvantageous, and therefore disclose sensors located adjacent to but outside of the carotid or femoral arteries.
The Cohen and Moore et al patents and the Konrad and Tacker articles are indicative of the types of devices with which sensors made and used according to the present invention may usefully be employed, and are all incorporated herein by reference in their entireties.